The next time your mom complains that you don't throw junk away, tell
her that you're in good company: nearly 15 million people suffer from
varying degrees of hoarding disorder. But what causes hoarding?

A few years ago, Samson (not his real name) unplugged his refrigerator.
It had, he says, “got out of hand.” He didn’t empty
it, and he hasn’t opened it since.

That's how Bonnie Tsui's journey to understanding the science of hoarding
began:

In a National Public Radio interview a couple of years ago, Frost talked
about the reasons hoarders might collect certain items: a decades-old
newspaper because it could be useful in the future; an array of bottle
caps purely for their fascinating physical characteristics; a seemingly
insignificant postcard because it reminded the owner of a loved one
or a specific event. Frost saw universality in the way the beliefs seem
to be tied to information processing. “There are some problems
with attention—that is, distractibility and sometimes a hyper
focus, problems with categorization, the ability to organize things,”
he explained. “People who hoard tend to live their lives visually
and spatially instead of categorically, like the rest of us do.”
One of his patients, Irene, would put an electricity bill on top of
a pile; if she needed it again, she would remember where it was in space,
rather than filing it away—mentally and physically—in a
“bills” category.
“We don’t know the nature of the emotional attachments that
people who hoard have to objects,” Frost told me. “How do
they form, and why are they so? What are the vulnerabilities that lead
up to it?”

Read the rest of Bonnie's article over at Pacific Standard Magazine:
Link

Nope... her father forcing her to clean and them not having as much money have helped some lol. Also she babysits her other grandchild (a toddler) every once in a while so can't keep the house gross and messy any longer. Or well... it's still messy, just the poop and pee thing isn't a problem any longer.

My mother-in-law is a hoarder. She's not as bad as she used to be, but it was HORRIBLE for a while. She was not a 'clean' hoarder and would let her pets poo and pee everywhere on everything. You could smell her house standing half a football field away... and I am not exaggerating.Thankfully her father moved back in to the home and basically forced his almost 60yo child to clean up the house.It's infuriating trying to deal with all of the mess, but she's not a bad person. Just stubborn and in denial.

I think you're right: hoarding is probably just the extreme end of the spectrum in the disorder. On the opposite end, there are those who clean compulsively (The article touched upon this, and why hoarding may be a unique disorder in and of itself, rather than just a continuation of OCD).